The use of diamond having high thermal conductivity in a heat spreader or a heat sink application is effective in enhancing heat dissipation of a semiconductor device. Further, a surface activated bonding method for bonding diamond to a semiconductor substrate of a semiconductor device has been proposed. In the surface activated bonding method, surfaces to be bonded are smoothed, the surfaces are irradiated with a noble gas beam so as to be activated, and subsequently, the surfaces are stacked on each other. Although minute unevenness still exit on the surfaces even if the surfaces are smoothed, because the target object is slightly deformed, the surfaces can be bonded to each other.
However, because diamond has extremely high rigidity, diamond does not deform. Therefore, many gaps are formed between a semiconductor substrate and diamond. Especially, when a SiC substrate is used as a semiconductor substrate, many gaps tends to be formed. Such gaps cause a decrease in bonding strength and an increase in interface thermal resistance, resulting in reduction in heat dissipation.